This invention relates generally to implosion protection bands for cathode ray tubes (CRT's) and particularly to a method of forming a shrink fit implosion protection band.
Cathode ray tubes are evacuated to a very low internal pressure and accordingly are subject to the possibility of implosion due to the stresses produced by atmospheric pressure acting on all surfaces of the tube. This problem has been addressed in the art by providing CRTs with implosion protection bands. Such bands are used to apply a compressive force to the side walls of the CRT to redistribute some of the faceplate forces. The redistribution of the faceplate forces decreases the probability of an implosion of the tube by minimizing tension in the corners of the faceplate. Implosion protection bands are also beneficial because they improve the impact resistance of the tube. Glass in compression is stronger than glass which is not in compression. The band causes compression in faceplate areas which otherwise are in tension. Additionally, in the event of an implosion the redistributed stresses cause the imploding glass to be directed toward the back of the cabinet in which the tube is mounted, thereby substantially reducing the probability of someone in the vicinity of the imploding tube being injured.
Implosion protection bands of the shrink fit type typically are manufactured by forming a strip of steel into a loop having the same configuration as the faceplate to be protected and joining the two ends of the strip on one side of the band. In some instances, the band is made by joining two identical strips on two sides of the loop. For both types of bands, the periphery of the loop is slightly smaller than the periphery of the faceplate. The lop is heated to approximately 300.degree. C. and 500.degree. C. and the coefficient of expansion of the material causes the loop to expand to dimensions permitting the loop to be slipped around the sides of the faceplate. As the band cools it shrinks and tightly surrounds the faceplate, thereby applying the necessary implosion protection compression to the faceplate sidewalls. The compressive force can be accurately controlled by accurately dimensioning the band because the coefficient of expansion of the banding material is known.
The ends of the strips are permanently joined by either welding or crimping. In either event, because the band is used to apply substantial pressure to the sidewalls of the tube it is essential that the joint formed when the two ends are coupled together be sufficiently strong to withstand the pressure. It is therefore important to test the integrity of the joint prior to applying the band to a CRT. It is also important to prepare the loop in a manner which assures that the loop will properly seat on the sidewalls of the CRT and will apply optimum compressive forces to the CRT. The present invention fulfills these important criteria.